[llvm] fix zlib buffer truncate edge cases and fix nits in tests

- add check before truncating (un)compressed data buffer if the buffer is already a perfect length, to avoid triggering truncate assertion in edge case.
- explictly coerce LLVM_ENABLE_ZLIB to a 0 or 1 value in OFF case, to match current ON, FORCE_ON behavior.
- fix code style nits in zlib tests

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D129698